Previous studies have shown lower grade gliomas with IDH mutation to have either 1p19q codeletion or a TP53 mutation.16 All tumors with a complete 1p19q codeletion, considered the hallmark of oligodendroglioma, also have a mutation in IDH1 or IDH2.45,46 The 1p19q codeletion is more common in IDH2-mutant tumors than in IDH1-mutant tumors (91% vs  48%, p < 0.001).55 The IDH-mutant and 1p19q codeleted status is diagnostic of oligodendroglioma even in the absence of any oligodendroglioma-looking tumor cells at microscopy. The gene discussed is IDH1; the disease is glioma.